Electrically conductive Kelvin contacts for microcircuit tester

ABSTRACT

Terminals of a device under test are connected to corresponding contact pads or leads by a series of electrically conductive contacts. Each terminal testing connects with both a “force” contact and a “sense” contact. In one embodiment, the sense contact partially or completely laterally surrounds the force contact, so that it need not have its own resiliency. The sense contact has a forked end with prongs that extend to opposite sides of the force contact. Alternatively, the sense contact surrounds the force contact and slides laterally to match a lateral translation component of a lateral cross-section of the force contact during longitudinal compression of the force contact. Alternatively, the sense contact includes rods that have ends on opposite sides of the force contact, and extend parallel.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority of provisional application No.61/171,141, filed on 21 Apr. 2009, provisional application No.61/257,236, filed on 2 Nov. 2009, and provisional application No.61/307,501, filed on 24 Feb. 2010, which hereby is incorporated hereinby reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to equipment for testingmicrocircuits.

2. Description of the Related Art

As microcircuits continually evolve to be smaller and more complex, thetest equipment that tests the microcircuits also evolves. There is anongoing effort to improve microcircuit test equipment, with improvementsleading to an increase in reliability, an increase in throughput, and/ora decrease in expense.

Mounting a defective microcircuit on a circuit board is relativelycostly. Installation usually involves soldering the microcircuit ontothe circuit board. Once mounted on a circuit board, removing amicrocircuit is problematic because the very act of melting the solderfor a second time ruins the circuit board. Thus, if the microcircuit isdefective, the circuit board itself is probably ruined as well, meaningthat the entire value added to the circuit board at that point is lost.For all these reasons, a microcircuit is usually tested beforeinstallation on a circuit board.

Each microcircuit must be tested in a way that identifies all defectivedevices, but yet does not improperly identify good devices as defective.Either kind of error, if frequent, adds substantial overall cost to thecircuit board manufacturing process, and can add retest costs fordevices improperly identified as defective devices.

Microcircuit test equipment itself is quite complex. First of all, thetest equipment must make accurate and low resistance temporary andnon-destructive electrical contact with each of the closely spacedmicrocircuit contacts. Because of the small size of microcircuitcontacts and the spacings between them, even small errors in making thecontact will result in incorrect connections. Connections to themicrocircuit that are misaligned or otherwise incorrect will cause thetest equipment to identify the device under test (DUT) as defective,even though the reason for the failure is the defective electricalconnection between the test equipment and the DUT rather than defects inthe DUT itself.

A further problem in microcircuit test equipment arises in automatedtesting. Testing equipment may test 100 devices a minute, or even more.The sheer number of tests cause wear on the tester contacts makingelectrical connections to the microcircuit terminals during testing.This wear dislodges conductive debris from both the tester contacts andthe DUT terminals that contaminates the testing equipment and the DUTsthemselves.

The debris eventually results in poor electrical connections duringtesting and false indications that the DUT is defective. The debrisadhering to the microcircuits may result in faulty assembly unless thedebris is removed from the microcircuits. Removing debris adds cost andintroduces another source of defects in the microcircuits themselves.

Other considerations exist as well. Inexpensive tester contacts thatperform well are advantageous. Minimizing the time required to replacethem is also desirable, since test equipment is expensive. If the testequipment is off line for extended periods of normal maintenance, thecost of testing an individual microcircuit increases.

Test equipment in current use has an array of test contacts that mimicthe pattern of the microcircuit terminal array. The array of testcontacts is supported in a structure that precisely maintains thealignment of the contacts relative to each other. An alignment plate orboard aligns the microcircuit itself with the test contacts. Many timesthe alignment plate is separate from the housing that houses thecontacts because it tends to wear and need replacing more often. Thetest housing and the alignment plate are mounted on a load board havingconductive pads that make electrical connection to the test contacts.The load board pads are connected to circuit paths that carry thesignals and power between the test equipment electronics and the testcontacts.

For the electrical tests, it is desired to form a temporary electricalconnection between each terminal on the device under test and acorresponding electrical pad on a load board. In general, it isimpractical to solder and remove each electrical terminal on themicrocircuit being contacted by a corresponding electrical probe on thetestbed. Instead of soldering and removing each terminal, the tester mayemploy a series of electrically conductive contacts arranged in apattern that corresponds to both the terminals on the device under testand the electrical pads on the load board. When the device under test isforced into contact with the tester, the contacts complete the circuitsbetween respective device under test contacts and corresponding loadboard pads. After testing, when the device under test is released, theterminals separate from the contacts and the circuits are broken.

The present application is directed to improvements to these contacts.

There is a type of testing known as “Kelvin” testing, which accuratelymeasures the resistance between two terminals on the device under test.Basically, Kelvin testing involves forcing a current to flow between thetwo terminals, measuring the voltage difference between the twoterminals, and using Ohm's Law to derive the resistance between theterminals, given as the voltage divided by the current. Each terminal onthe device under test is electrically connected to two contacts andtheir associated pads on the load board. One of the two pads supplies aknown amount of current. The other pad, known as the “sense” connection,is a high-impedance connection that acts as a voltmeter, which does notdraw any significant amount of current. In other words, each terminal onthe device under test that is to undergo Kelvin testing issimultaneously electrically connected to two pads on the load board—onepad supplying a known amount of current and the other pad measuring avoltage and drawing an insignificant amount of current while doing so.The terminals are Kelvin tested two at a time, so that a singleresistance measurement uses two terminals on the load board and fourcontact pads.

In this application, the contacts that form the temporary electricalconnections between the device under test and the load board may be usedin several manners. In a “standard” test, each contact connects aparticular terminal on the device under test to a particular pad on theload board, with the terminals and pads being in a one-to-onerelationship. For these standard tests, each terminal corresponds toexactly one pad, and each pad corresponds to exactly one terminal. In a“Kelvin” test, there are two contacts contacting each terminal on thedevice under test, as described above. For these Kelvin tests, eachterminal on the device under test corresponds to two pads on the loadboard, and each pad on the load board corresponds to exactly oneterminal on the device under test. Although the testing scheme may vary,the mechanical structure and use of the contacts is essentially thesame, regardless of the testing scheme.

There are many aspects of the testbeds that may be incorporated fromolder or existing testbeds. For instance, much of the mechanicalinfrastructure and electrical circuitry may be used from existing testsystems, and may be compatible with the electrically conductive contactsdisclosed herein. Such existing systems are listed and summarized below.

An exemplary microcircuit tester is disclosed in United States PatentApplication Publication Number US 2007/0202714 A1, titled “Test contactsystem for testing integrated circuits with packages having an array ofsignal and power contacts”, invented by Jeffrey C. Sherry, published onAug. 30, 2007 and incorporated by reference herein in its entirety.

For the tester of '714, a series of microcircuits is testedsequentially, with each microcircuit, or “device under test”, beingattached to a testbed, tested electrically, and then removed from thetestbed. The mechanical and electrical aspects of such a testbed aregenerally automated, so that the throughput of the testbed may be keptas high as possible.

In '714, a test contact element for making temporary electrical contactwith a microcircuit terminal comprises at least one resilient fingerprojecting from an insulating contact membrane as a cantilevered beam.The finger has on a contact side thereof, a conducting contact pad forcontacting the microcircuit terminal. Preferably the test contactelement has a plurality of fingers, which may advantageously have apie-shaped arrangement. In such an arrangement, each finger is definedat least in part by two radially oriented slots in the membrane thatmechanically separate each finger from every other finger of theplurality of fingers forming the test contact element.

In '714, a plurality of the test contact elements can form a testcontact element array comprising the test contact elements arranged in apredetermined pattern. A plurality of connection vias are arranged insubstantially the predetermined pattern of the test contacts elements,with each of said connection vias is aligned with one of the testcontact elements. Preferably, an interface membrane supports theplurality of connection vias in the predetermined pattern. Numerous viascan be embedded into the pie pieces away from the device contact area toincrease life. Slots separating fingers could be plated to create anI-beam, thereby preventing fingers from deforming, and also increasinglife.

The connection vias of '714 may have a cup shape with an open end, withthe open end of the cup-shaped via contacting the aligned test contactelement. Debris resulting from loading and unloading DUTs from the testequipment can fall through the test contact elements where thecup-shaped vias impound the debris.

The contact and interface membranes of '714 may be used as part of atest receptacle including a load board. The load board has a pluralityof connection pads in substantially the predetermined pattern of thetest contacts elements. The load board supports the interface membranewith each of the connection pads on the load board substantially alignedwith one of the connection vias and in electrical contact therewith.

In '714, the device uses a very thin conductive plate with retentionproperties that adheres to a very thin non-conductive insulator. Themetal portion of the device provides multiple contact points or pathsbetween the contacting I/O and the load board. This can be done eitherwith a plated via hole housing or with plated through hole vias, orbumped surfaces, possibly in combination with springs, that has thefirst surface making contact with the second surface, i.e., the deviceI/O. The device I/O may be physically close to the load board, thusimproving electrical performance.

One particular type of microcircuit often tested before installation hasa package or housing having what is commonly referred to as a ball gridarray (BGA) terminal arrangement. A typical BGA package may have theform of a flat rectangular block, with typical sizes ranging from 5 mmto 40 mm on a side and 1 mm thick.

A typical microcircuit has a housing enclosing the actual circuitry.Signal and power (S&P) terminals are on one of the two larger, flatsurfaces, of the housing. Typically, terminals occupy most of the areabetween the surface edges and any spacer or spacers. Note that in somecases, a spacer may be an encapsulated chip or a ground pad.

Each of the terminals may include a small, approximately sphericalsolder ball that firmly adheres to a lead from the internal circuitrypenetrating surface, hence the term “ball grid array.” Each terminal andspacer project a small distance away from the surface, with theterminals projecting farther from the surface than the spacers. Duringassembly, all terminals are simultaneously melted, and adhere tosuitably located conductors previously formed on the circuit board.

The terminals themselves may be quite close to each other. Some havecenterline spacings of as little as 0.25 mm, and even relatively widelyspaced terminals may still be around 1.5 mm apart. Spacing betweenadjacent terminals is often referred to as “pitch.”

In addition to the factors mentioned above, BGA microcircuit testinginvolves additional factors.

First, in making the temporary contact with the ball terminals, thetester should not damage the S&P terminal surfaces that contact thecircuit board, since such damage may affect the reliability of thesolder joint for that terminal.

Second, the testing process is more accurate if the length of theconductors carrying the signals is kept short. An ideal test contactarrangement has short signal paths.

Third, solders commonly in use today for device terminals are mainly tinfor environmental purposes. Tin-based solder alloys are likely todevelop an oxide film on the outer surface that conducts poorly. Oldersolder alloys include substantial amounts of lead, which do not formoxide films. The test contacts must be able to penetrate the oxide filmpresent.

BGA test contacts currently known and used in the art employ springcontacts made up of multiple pieces including a spring, a body and topand bottom plungers.

United States Patent Application Publication No. US 2003/0192181 A1,titled “Method of making an electronic contact” and published on Oct.16, 2003, shows microelectronic contacts, such as flexible, tab-like,cantilever contacts, which are provided with asperities disposed in aregular pattern. Each asperity has a sharp feature at its tip remotefrom the surface of the contact. As mating microelectronic elements areengaged with the contacts, a wiping action causes the sharp features ofthe asperities to scrape the mating element, so as to provide effectiveelectrical interconnection and, optionally, effective metallurgicalbonding between the contact and the mating element upon activation of abonding material.

According to United States Patent Application Publication No. US2004/0201390 A1, titled “Test interconnect for bumped semiconductorcomponents and method of fabrication” and published on Oct. 14, 2004, aninterconnect for testing semiconductor components includes a substrate,and contacts on the substrate for making temporary electricalconnections with bumped contacts on the components. Each contactincludes a recess and a pattern of leads cantilevered over the recessconfigured to electrically engage a bumped contact. The leads areadapted to move in a z-direction within the recess to accommodatevariations in the height and planarity of the bumped contacts. Inaddition, the leads can include projections for penetrating the bumpedcontacts, a non-bonding outer layer for preventing bonding to the bumpedcontacts, and a curved shape which matches a topography of the bumpedcontacts. The leads can be formed by forming a patterned metal layer onthe substrate, by attaching a polymer substrate with the leads thereonto the substrate, or by etching the substrate to form conductive beams.

According to U.S. Pat. No. 6,246,249 B1, titled “Semiconductorinspection apparatus and inspection method using the apparatus” andissued on Jun. 12, 2001 to Fukasawa, et al., a semiconductor inspectionapparatus performs a test on a to-be-inspected device which has aspherical connection terminal. This apparatus includes a conductor layerformed on a supporting film. The conductor layer has a connectionportion. The spherical connection terminal is connected to theconnection portion. At least a shape of the connection portion ischangeable. The apparatus further includes a shock absorbing member,made of an elastically deformable and insulating material, for at leastsupporting the connection portion. A test contact element of theinvention for making temporary electrical contact with a microcircuitterminal comprises at least one resilient finger projecting from aninsulating contact membrane as a cantilevered beam. The finger has on acontact side thereof, a conducting contact pad for contacting themicrocircuit terminal.

In U.S. Pat. No. 5,812,378, titled “Microelectronic connector forengaging bump leads” and issued on Sep. 22, 1998 to Fjelstad, et al., aconnector for microelectronic includes a sheet-like body having aplurality of holes, desirably arranged in a regular grid pattern. Eachhole is provided with a resilient laminar contact such as a ring of asheet metal having a plurality of projections extending inwardly overthe hole of a first major surface of the body. Terminals on a secondsurface of the connector body are electrically connected to thecontacts. The connector can be attached to a substrate such amulti-layer circuit panel so that the terminals on the connector areelectrically connected to the leads within the substrate.Microelectronic elements having bump leads thereon may be engaged withthe connector and hence connected to the substrate, by advancing thebump leads into the holes of the connector to engage the bump leads withthe contacts. The assembly can be tested, and if found acceptable, thebump leads can be permanently bonded to the contacts.

According to United States Patent Application Publication No. US2001/0011907 A1, titled “Test interconnect for bumped semiconductorcomponents and method of fabrication” and published on Aug. 9, 2001, aninterconnect for testing semiconductor components includes a substrate,and contacts on the substrate for making temporary electricalconnections with bumped contacts on the components. Each contactincludes a recess and a support member over the recess configured toelectrically engage a bumped contact. The support member is suspendedover the recess on spiral leads formed on a surface of the substrate.The spiral leads allow the support member to move in a z-directionwithin the recess to accommodate variations in the height and planarityof the bumped contacts. In addition, the spiral leads twist the supportmember relative to the bumped contact to facilitate penetration of oxidelayers thereon. The spiral leads can be formed by attaching a polymersubstrate with the leads thereon to the substrate, or by forming apatterned metal layer on the substrate. In an alternate embodimentcontact, the support member is suspended over the surface of thesubstrate on raised spring segment leads.

Consider an electrical chip that is manufactured to be incorporated intoa larger system. When in use, the chip electrically connects the deviceto the larger system by a series of contacts or terminals. For instance,the contacts on the electrical chip may plug into corresponding socketsin a computer, so that the computer circuitry may electrically connectwith the chip circuitry in a predetermined manner. An example of such achip may be a memory card or processor for a computer, each of which maybe insertable into a particular slot or socket that makes one or moreelectrical connections with the chip.

It is highly desirable to test these chips before they are shipped, orbefore they are installed into other systems. Such component-leveltesting may help diagnose problems in the manufacturing process, and mayhelp improve system-level yields for systems that incorporate the chips.Therefore, sophisticated test systems have been developed to ensure thatthe circuitry in the chip performs as designed. The chip is attached tothe tester, as a “device under test”, is tested, and is then detachedfrom the tester. In general, it is desirable to perform the attachment,testing, and detachment as rapidly as possible, so that the throughputof the tester may be as high as possible.

The test systems access the chip circuitry through the same contacts orterminals that will later be used to connect the chip in its finalapplication. As a result, there are some general requirements for thetest system that perform the testing. In general, the tester shouldestablish electrical contact with the various contacts or terminals sothat the contacts are not damaged, and so that a reliable electricalconnection is made with each contact.

Most testers of this type use mechanical contacts between the chip I/Ocontacts and the tester contacts, rather than soldering and de-solderingor some other attachment method. When the chip is attached to thetester, each contact on the chip is brought into mechanical andelectrical contact with a corresponding pad on the tester. Aftertesting, the chip is removed from the tester, and the mechanical andelectrical contacts are broken.

In general, it is highly desirable that the chip and the tester bothundergo as little damage as possible during the attachment, testing, anddetachment procedures. Pad layouts on the tester may be designed toreduce or minimize wear or damage to the chip contacts. For instance, itis not desirable to scrape the device I/O (leads, contacts, pads orballs), bend or deflect the I/O, or perform any operation that mightpermanently change or damage the I/O in any way. Typically, the testersare designed to leave the chips in a final state that resembles theinitial state as closely as possible. In addition, it is also desirableto avoid or reduce any permanent damage to the tester or tester pads, sothat tester parts may last longer before replacement.

There is currently a great deal of effort spent by tester manufacturerson the pad layouts. For instance, the pads may include a spring-loadmechanism that receives the chip contacts with a prescribed resistingforce. In some applications, the pads may have an optional hard stop atthe extreme end of the spring-load force range of travel. The goal ofthe pad layout is to establish a reliable electrical connection with thecorresponding chip contacts, which may be as close as possible to a“closed” circuit when the chip is attached, and may be as close aspossible to an “open” circuit when the chip is detached.

Because it is desirable to test these chips as quickly as possible, orsimulate their actual use in a larger system, it may be necessary todrive and/or receive electrical signals from the contacts at very highfrequencies. The test frequencies of current-day testers may be up to 40GHz or more, and the test frequencies are likely to increase with futuregeneration testers.

For low-frequency testing, such as that done close to DC (0 Hz), theelectrical performance may be handled rather simplistically: one wouldwant an infinitely high resistance when the chip is detached, and aninfinitesimally small resistance when the chip is attached.

At higher frequencies, other electrical properties come into play,beyond just resistance. Impedance (or, basically, resistance as afunction of frequency) becomes a more proper measure of electricalperformance at these higher frequencies. Impedance may include phaseeffects as well as amplitude effects, and can also incorporate andmathematically describe the effects of resistance, capacitance andinductance in the electrical path. In general, it is desirable that thecontact resistance in the electrical path formed between the chip I/Oand the corresponding pad on the load card be sufficiently low, whichmaintains a target impedance of 50 ohms, so that the tester itself doesnot significantly distort the electrical performance of the chip undertest. Note that most test equipment is designed to have 50 ohm input andoutput impedances.

For modern-day chips that have many, many closely spaced I/O, it becomeshelpful to simulate the electrical and mechanical performance at thedevice I/O interface. Finite-element modeling in two- or threedimensions has become a tool of choice for many designers. In someapplications, once a basic geometry style has been chosen for the testerpad configuration, the electrical performance of the pad configurationis simulated, and then the specific sizes and shapes may be iterativelytweaked until a desired electrical performance is achieved. For theseapplications, the mechanical performance may be determined almost as anafterthought, once the simulated electrical performance has reached aparticular threshold.

BRIEF SUMMARY OF THE INVENTION

An embodiment is a device for forming a plurality of temporarymechanical and electrical connections between a device under test havinga plurality of terminals and a load board having a plurality of contactpads, each contact pad being laterally arranged to correspond to exactlyone terminal, comprising: a laterally oriented, electrically insulatinghousing longitudinally adjacent to the contact pads on the load board; aplurality of electrically conductive force contacts extending throughlongitudinal holes in the housing toward the device under test and beingcompressible/deflectable through the holes in the housing, each forcecontact in the plurality being laterally arranged to correspond toexactly one terminal; and a plurality of electrically conductive sensecontacts, each sense contact in the plurality being laterally arrangedto correspond to exactly one force contact and exactly one terminal,each sense contact in the plurality extending toward the device undertest proximate the corresponding force contact. Each sense contact inthe plurality includes a fixed portion, a free portion extendinghingedly away from the housing, and a hinged portion connecting thefixed portion and the free portion. The hinged portion is laterallyseparated from the corresponding force contact. The free portionincludes a forked portion at its distal end that extends on oppositesides of a distal end of the corresponding force contact. The fixedportion is meant to indicate that it is a point along the contact whereflexure is limited for prevented. The location of the fixed portion orpoint relative to the tip determines flexure, all other things beingequal.

An additional embodiment is a device for forming a plurality oftemporary mechanical and electrical connections between a device undertest having a plurality of terminals and a load board having a pluralityof contact pads, each contact pad being laterally arranged to correspondto exactly one terminal, comprising: a laterally oriented, electricallyinsulating housing longitudinally adjacent to the contact pads on theload board; a plurality of electrically conductive force contactsextending through longitudinal holes in the housing toward the deviceunder test and being compressible/deflectable through the holes in thehousing, the compressibility including a lateral translation of alateral cross-section of each force contact, each force contact in theplurality being laterally arranged to correspond to exactly oneterminal; and a plurality of electrically conductive sense contacts,each sense contact in the plurality being laterally arranged tocorrespond to exactly one force contact and exactly one terminal, eachsense contact in the plurality laterally surrounding the correspondingforce contact and being horizontal/laterally slidable along the housing,the horizontal/lateral sliding corresponding to the horizontal lateraltranslation of the horizontal/lateral cross-section of the correspondingforce contact such as in FIG. 8.

A further embodiment is a device for forming a plurality of temporarymechanical and electrical connections between a device under test havinga plurality of terminals and a load board having a plurality of contactpads, each contact pad being laterally arranged to correspond to exactlyone terminal, comprising: a laterally oriented, electrically insulatinghousing longitudinally adjacent to the contact pads on the load board; aplurality of electrically conductive force contacts extending throughlongitudinal holes in the housing toward the device under test and beingcompressible/deflectable through the holes in the housing, each forcecontact in the plurality being laterally arranged to correspond toexactly one terminal; and a plurality of electrically conductive sensecontacts, each sense contact in the plurality being laterally arrangedto correspond to exactly one force contact and exactly one terminal.Each sense contact in the plurality includes a pair of electricallyconductive rods extending generally laterally along the housing. Thepair of electrically conductive rods fit within corresponding channelsin the electrically insulating housing. Each conductive rod in the pairhas a distal end bent out of the plane of the housing toward the deviceunder test to hit exposed I/O pads under the device under test. The twodistal ends in each sense pair of rods are directly adjacent to and areon opposite sides of the corresponding force contact.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a side-view drawing of a portion of the test equipment forreceiving a device under test (DUT), for standard electrical testing.

FIG. 2 is a side-view drawing of the test equipment of FIG. 1, with theDUT electrically engaged.

FIG. 3 is a side-view drawing of a portion of the test equipment forreceiving a device under test (DUT), for Kelvin testing.

FIG. 4 is a side-view drawing of the test equipment of FIG. 3, with theDUT electrically engaged.

FIG. 5 is a plan drawing of a first design of force and sense contactson the test equipment.

FIG. 6 is a plan drawing of a second design of force and sense contactson the test equipment.

FIG. 7 is a plan drawing of a third design of force and sense contactson the test equipment.

FIG. 8 is a plan drawing of a fourth design of force and sense contactson the test equipment.

FIG. 9 is a plan drawing of a fifth design of force and sense contactson the test equipment.

FIG. 10 is a plan drawing of a sixth design of force and sense contactson the test equipment.

FIG. 11 is a plan drawing of a seventh design of force and sensecontacts on the test equipment.

FIG. 12 is a side-view drawing of two sets of terminal/contacts, for thetest equipment of FIGS. 3 and 4, with the DUT electrically engaged.

FIG. 13 is side-view cross-sectional drawing of a sample geometry of asense (voltage) contact in its path from the terminal on the deviceunder test to the contact pad on the load board.

FIG. 14 is side-view cross-sectional drawing of another sample geometryof a sense (voltage) contact in its path from the terminal on the deviceunder test to the contact pad on the load board.

FIG. 15 is a side-view schematic drawing of a pair of sense contactshaving tips that are angled away from the central force (current)contact.

FIG. 16 is a top-view schematic drawing of a pair of sense contactsthat, at their distal ends, include laterally-extending portions thatextend toward each other.

FIG. 17 is a top-view schematic drawing of a pair of sense contactsthat, at their distal ends, include laterally-extending portions thatextend toward each other.

FIG. 18 is a top-view schematic drawing of a single sense contact that,at its distal end, includes a laterally-extending portion that extendspartway around the force contact.

FIG. 19 is a top-view schematic drawing of a single sense contact that,at its distal end, includes a laterally-extending portion that does notextend partway around the force contact.

FIG. 20 is a side-view schematic drawing of a pair of sense contactshaving tips that are angled toward each other and cross each other overor alongside the central force contact.

FIG. 21 is a top-view schematic drawing of a pair of sense contactsthat, at their distal ends, include laterally-extending portions thatextend up out of the page.

FIG. 22 is a perspective schematic view of a leaded integrated circuitpackage and the Kelvin contact system therefore.

FIG. 23 is a close up perspective of the system of FIG. 22 with portionsremoved for clarity.

FIG. 24, is a view like FIG. 23 but taken from the other side.

FIG. 25 is a side schematic view of the system applied to a leadeddevice in an compressed state.

FIG. 26 is a view like FIG. 25 except with the elastomeric portionsremoved.

FIG. 27 is a view like FIG. 25 except with a compressed state and sensecontact designed to hit only front portion of lead protruding fromdevice.

FIG. 28 is a view like FIG. 27 except taken from the other side andapproach has a tines (finger) bent upward to initialize contact with thedevice sooner to provide more compliance.

FIG. 29 is a schematic view showing both uncompressed and compressedstates simultaneously of concept that only connects the from portion ofthe device lead.

FIG. 30 is a view like FIG. 29, showing uncompressed and compressedstates simultaneously with dual tine concept where sense fork tinesstraddle force contact.

FIG. 31 is a perspective view showing a forked sense lead straddling areduced tip thickness force contact.

FIG. 32 is a perspective view like FIG. 31 showing a single sided senselead and a force contact with an offset.

FIG. 33 is a perspective view similar to FIG. 29.

DETAILED DESCRIPTION OF THE INVENTION

A general summary of the disclosure follows.

The terminals of a device under test are temporarily electricallyconnected to corresponding contact pads on a load board by a series ofelectrically conductive contacts. The terminals may be pads, balls,wires (leads) or other contact points. Each terminal that undergoesKelvin testing connects with both a “force” contact and a “sense”contact, with each contact electrically connecting to a respective,single contact pad on the load board. The force contact delivers a knownamount of current to or from the terminal, and the sense contactmeasures a voltage at the terminal and draws a negligible amount ofcurrent to or from the terminal. The sense contact partially orcompletely laterally surrounds the force contact, so that it need nothave its own resiliency, though it may also be resilient in its ownright. This helps keep the force contact in alignment by preventinglateral wobbling. In a first case, the sense contact has a forked endwith prongs that extend to opposite sides of the force contact. In asecond case, the sense contact completely laterally surrounds the forcecontact and slides horizontally/laterally to match a horizontaltranslation component of a horizontal cross-section of the force contactduring vertical compression of the force contact. In a third case, thesense contact includes two rods that have ends on opposite sides of theforce contact, and extend parallel and laterally away from the forcecontact. In these cases, the sense contact extends horizontally along amembrane or housing that supports the force contacts. The rods may behoused in respective channels along the membrane or in the housing.

The preceding paragraph is merely a summary of the disclosure, andshould not be construed as limiting in any way. The test device isdescribed in much greater detail below.

FIGS. 1 and 2 show a tester that performs conventional electricaltesting, in which there is one-to-one correspondence between theterminals on the device under test and the contact pads on the loadboard. In contrast, FIGS. 3 and 4 show a tester that performs Kelvintesting, in which there are two contact pads on the load board connectedto each terminal on the device under test. Despite their differencesbetween conventional and Kelvin testing, the tester elements have agreat deal in common. As such, they are first described with regard toFIGS. 1 and 2. Following their description for conventional testing, theelements are then described as used with Kelvin testing, as shown inFIGS. 3 and 4. Differences between the cases are highlighted at thispoint in the description.

FIG. 1 is a side-view drawing of a portion of the test equipment forreceiving a device under test (DUT) 1, for conventional electricaltesting. The DUT 1 is placed onto the tester 5, electrical testing isperformed, and the DUT 1 is then removed from the tester 5. Anyelectrical connections are made by pressing components into electricalcontact with other components; there is no soldering or de-soldering atany point in the testing of the DUT 1.

The entire electrical test procedure may only last about a fraction of asecond, so that rapid, accurate placement of the device under test 1becomes important for ensuring that the test equipment is usedefficiently. The high throughput of the tester 5 usually requiresrobotic handling of the devices under test 1. In most cases, anautomated mechanical system places the DUT 1 onto the tester 5 prior totesting, and removes the DUT 1 once testing has been completed. Thehandling and placement mechanism may use mechanical and optical sensorsto monitor the position of the DUT 1, and a combination of translationand rotation actuators to align and place the DUT 1 on the testbed. Suchautomated mechanical systems are mature and have been used in many knownelectrical testers; these known robotic systems may also be used withany or all of the tester elements disclosed herein. Alternatively, theDUT 1 may be placed by hand, or placed by a combination of hand-fed andautomated equipment.

Likewise, the electrical algorithms that are used to test each terminalon the DUT 1 are well established, and have been used in many knownelectrical testers. These known electrical algorithms may also be usedwith any or all of the tester elements disclosed herein.

The device under test 1 typically includes one or more devices, andincludes signal and power terminals that connect to the device. Thedevice and terminals may be on one side of the device under test 1, ormay be on both sides of the device under test 1. For use in the tester5, all the terminals 2 should be accessible from one side of the deviceunder test 1, although it will be understood that there may be one ormore elements on the opposite side of the device under test 1, or thatthere may be other elements and/or terminals on the opposite side thatmay not be tested by accessing terminals 2.

Each terminal 2 is formed as a small, pad on button side of device orpossibly a lead protruding from the body of the device. Prior totesting, the pad or lead 2 is attached to an electrical lead thatconnects internally to other leads, to other electrical components,and/or to one or more chips in the device under test 1. The volume andsize of the pads or leads may be controlled quite precisely, and thereis typically not much difficulty caused by pad-to-pad or lead-to-leadsize variations or placement variations. During testing, the terminals 2remain solid, and there is no melting or re-flowing of any solder 2.

The terminals 2 may be laid out in any suitable pattern on the surfaceof the device under test 1. In some cases, the terminals 2 may be in agenerally square grid, which is the origin of an expression thatdescribes the device under test 1, QFN, DFN, MLF or QFP for leadedparts. There may also be deviations away from a rectangular grid,including irregular spacings and geometries. It will be understood thatthe specific locations of the terminals may vary as needed, withcorresponding locations of pads on the load board and contacts on themembrane or housing being chosen to match those of the device under testterminals 2. In general, the spacing between adjacent terminals 2 is inthe range of 0.25 to 1.5 mm, with the spacing being commonly referred toas a “pitch”.

When viewed from the side, as in FIG. 1, the device under test 1displays a line of terminals 2, which may optionally include gaps andirregular spacings. These terminals 2 are made to be generally planar,or as planar as possible with typical manufacturing processes. In manycases, if there are chips or other elements on the device under test 1,the protrusion of the chips is usually less than the protrusion of theterminals 2 away from the device under test 1.

The tester 5 of FIG. 1 includes a load board 3.

The load board 3 includes a load board substrate 6 and circuitry that isused to test electrically the device under test 1. Such circuitry mayinclude driving electronics that can produce one or more AC voltageshaving one or more particular frequencies, and detection electronicsthat can sense the response of the device under test 1 to such drivingvoltages. The sensing may include detection of a current and/or voltageat one or more frequencies. Such driving and sensing electronics is wellknown in the industry, and any suitable electronics from known testersmay be used with the tester elements disclosed herein.

In general, it is highly desirable that the features on the load board3, when mounted, are aligned with corresponding features on the deviceunder test 1. Typically, both the device under test 1 and the load board3 are mechanically aligned to one or more locating features on thetester 5. The load board 3 may include one or more mechanical locatingfeatures, such as fiducials or precisely-located holes and/or edges,which ensure that the load board 3 may be precisely seated on the tester5. These locating features typically ensure a lateral alignment (x, y)of the load board, and/or a longitudinal alignment (z) as well. Themechanical locating features are well known in the industry, and anysuitable electronics from known testers may be used with the testerelements disclosed herein. The mechanical locating features are notshown in FIG. 1.

In general, the load board 3 may be a relatively complex and expensivecomponent. In many cases, it may be advantageous to introduce anadditional, relatively inexpensive element into the tester 5 thatprotects the contact pads 4 of the load board 3 from wear and damage.Such an additional element may be an interposer membrane 10. Theinterposer membrane 10 also mechanically aligns with the tester 3 withsuitable locating features (not shown), and resides in the tester 5above the load board 3, facing the device under test 1.

The interposer membrane 10 includes a series of electrically conductivecontacts 20, which extend longitudinally outward on either side of themembrane 10. Each contact 20 may include a resilient element, such as aspring or an elastomer material, and is capable of conducting anelectrical current to/from the load board from/to the device under testwith sufficiently low resistance or impedance. Each contact may be asingle conductive unit, or may alternatively be formed as a combinationof conductive elements.

In general, each contact 20 connects one contact pad 4 on the load board3 to one terminal 2 on the device under test 1, although there may betesting schemes in which multiple contact pads 4 connect to a singleterminal 2, or multiple terminals 2 connect to a single contact pad 4.For simplicity, we assume in the text and drawings that a single contact20 connects a single pad to a single terminal, although it will beunderstood that any of the tester elements disclosed herein may be usedto connect multiple contact pads connect to a single terminal, ormultiple terminals to a single contact pad. Typically, the interposermembrane 10 electrically connects the load board pads and the bottomcontact surface of the test contactor. It may alternatively be used toconvert an existing load board pad configuration to a vehicle, which isa test socket used to connect and test a device under test.

Although the interposer membrane 10 may be removed and replacedrelatively easily, compared with removal and replacement of the loadboard 3, we consider the interposer membrane 10 to be part of the tester5 for this document. During operation, the tester 5 includes the loadboard 3, the interposer membrane 10, and the mechanical constructionthat mounts them and holds them in place (not shown). Each device undertest 1 is placed against the tester 5, is tested electrically, and isremoved from the tester 5.

A single interposer membrane 10 may test many devices under test 1before it wears out, and may typically last for several thousand testsor more before requiring replacement. In general, it is desirable thatreplacement of the interposer membrane 10 be relatively fast and simple,so that the tester 5 experiences only a small amount of down time formembrane replacement. In some cases, the speed of replacement for theinterposer membrane 10 may even be more important than the actual costof each membrane 10, with an increase in tester up-time resulting in asuitable cost savings during operation.

FIG. 1 shows the relationship between the tester 5 and the devices undertest 1. When each device 1 is tested, it is placed into a suitablerobotic handler with sufficiently accurate placement characteristics, sothat a particular terminal 2 on the device 1 may be accurately andreliably placed (in x, y and z) with respect to corresponding contacts20 on the interposer membrane 10 and corresponding contact pads 4 on theload board 3.

The robotic handler (not shown) forces each device under test 1 intocontact with the tester 5. The magnitude of the force depends on theexact configuration of the test, including the number of terminals 2being tested, the force to be used for each terminal, typicalmanufacturing and alignment tolerances, and so forth. In general, theforce is applied by the mechanical handler of the tester (not shown),acting on the device under test 1. In general, the force is generallylongitudinal, and is generally parallel to a surface normal of the loadboard 3.

FIG. 2 shows the tester and device under test 1 in contact, withsufficient force being applied to the device under test 1 to engage thecontacts 20 and form an electrical connection 9 between each terminal 2and its corresponding contact pad 4 on the load board 3. As statedabove, there may alternatively be testing schemes in which multipleterminals 2 connect to a single contact pad 4, or multiple contact pads4 connect to a single terminal 2, but for simplicity in the drawings weassume that a single terminal 2 connects uniquely to a single contactpad 4.

FIGS. 1 and 2 above show conventional electrical testing, whichessentially answers the question, “Is Terminal A electrically connectedat all to Terminal B?” Currents are driven from the load board to aparticular terminal on the device under test, pass internally in thedevice under test to another terminal, then return to the load board.

In contrast with conventional electrical testing, Kelvin testingessentially answers the question, “What is the electrical resistancebetween Terminal A and Terminal B?” As with conventional testing,currents are driven from the load board to a terminal, internally toanother terminal, and back to the load board. However, in Kelvintesting, each terminal electrically contacts two contactssimultaneously. One contact in the pair supplies the known amount ofcurrent (I), as is done in conventional testing, while the other contactin the pair measures a voltage (V) without drawing a significant amountof current. From known amounts of current (I) and voltage (V), Ohm's Law(V=IR) may be used to determine the resistance R (=V/I) between twoparticular terminals on the load board.

The force or “current” contact may be considered a low-resistance orlow-impedance contact, while the sense or “voltage” contact may beconsidered a high-resistance or high-impedance contact. Note that atypical voltmeter operates in a manner similar to that of thehigh-resistance sense or “voltage” contacts.

FIGS. 3 and 4 show a tester that performs Kelvin testing. Many of theelements are analogous to those of the conventional tester shown inFIGS. 1 and 2, and are numbered accordingly.

Note that there is a pair of contact pads 4 for every terminal 2, withone in the pair for current, and the other for voltage. There is also apair of contacts 20 for every terminal 2 and every pair of contact pads4, with each contact electrically connecting a contact pad 4 to thecorresponding terminal 2. Note that the two contacts in each pair aregenerally electrically insulated from each other, and form electricalconnections 9 between the terminal 2 and the contact pad 4. FIG. 12shows a close-up view of two pairs of terminal/contacts, for the testequipment of FIGS. 3 and 4.

In the schematic drawings of FIGS. 3 and 4, the contacts 20 are drawn asbeing similar in shape and size, and being located adjacent to eachother, so that the terminal makes contact with both contacts at the sametime. While this may be sufficient from an electrical point of view,mechanically it leaves much to be desired. For instance, the terminalmay be laterally misaligned with respect to the contact pair, so thatthe terminal contacts one contact but misses the other. In addition, amembrane having such a Kelvin testing scheme may be far moremechanically complicated than a comparable conventional testing method,because the number of contacts is essentially doubled, while the lateralarea for the contacts remains the same. In general, it is mechanicallychallenging to fit so many contacts in so small an area, due to the tinysize of the parts, and the need for springs, elastomers, or some othermechanical resistance-producing device for each contact to generate zaxis compliance. As a result, there exists a need for an improvedmechanical layout for the electrical scheme shown in FIGS. 3 and 4. Theremainder of this document addresses such a need, and presents variousmechanical layouts that are improvements over the side-by-side design ofFIGS. 3 and 4.

One simplifying feature is relying mainly or solely on the force(current) contact for resiliency, i.e., the springiness or resistingforce that pushes back on the terminal when the device under test isforced into contact with the tester. This reduces the mechanicalcomplexity required for the sense (voltage) contact.

In addition, in some cases, the sense contact may have less strictelectrical requirements than the force contact, since the purpose of thesense contact is to measure voltage without drawing a significant amountof current. Such a low current flow may allow the sense contact to bethinner than the force contact, and may allow the sense contact to bendinto a variety of shapes and orientations. Some of these shapes may beacceptable for the sense contact, but might show unacceptablehigh-frequency performance if they were used for the moreelectrically-demanding force contact.

Removing the resiliency from the sense contact and relaxing the criteriaon electrical performance may allow for a variety of possibleorientations and shapes for the sense contact.

For example, one end of the sense contact may lie adjacent to the topend of the force contact. The sense contact may then extend generallylaterally along the top surface of the interposer or housing (sometimescalled membrane), may curve downward through a hole in the housing, andmay contact the corresponding contact pad on the load board afterpassing through the housing.

The design 50 of FIG. 5 shows a portion of an exemplary housing 51, anarray of holes 53 through the housing 51 that are laterally arranged tocorrespond to terminals 2 on the device under test 1, two exemplaryforce (current) contacts protruding upward (toward the device under test1) through the holes 53, two exemplary sense (voltage) contacts 54extending laterally away from the top of the force contacts 52, and twoexemplary terminals 2 on the device under test 1 that each contact botha force contact 52 and a sense contact 54. The leftmost exemplaryterminal 2 corresponds to when the device under test 1 is just barelycontacting the tester 5, and the rightmost exemplary terminal 2corresponds to when the device under test 1 is forced into contact withthe tester 5.

On each force contact 52, there is a notch removed from the top end thataccommodates a portion of the far end of the sense contact 54. When thedevice under test 1 is forced into contact with the tester 5, eachterminal 2 mechanically and electrically contacts both the respectiveforce contact 52 and the respective sense contact 54. Contact 54 has aplanar arm 54 a, an extension 54 b which rises from the housing surfacepreferably along a line to contact point 54 c. The extension 54 b mayalso be arcuate, concave or convex. The point of contact 54 c ispreferably at the sharp corner intersection at its end. The sharp cornerassists in removing oxide on the terminal 2 during insertion.

For the case in which there is little or no contact force applied (theleftmost terminal 2 shown in FIG. 5), the force contact 52 protrudesupward under the influence of its own resiliency. A portion of the farend of the sense contact 54 is bent upwards as well so that its tines 54a (in FIG. 6) are angled at 20-30 degrees (ie 20 degrees or 21 . . . 30degrees) from the plane of portion 64. Tines 54 a are also taperinwardly toward the force contact 62, preferably in a straight line toform a triangular tip (FIG. 6) or rectangular (FIG. 5) but may alsofollow an arcuate line toward the tip. The sense contact 54 may have afixed portion, attached to or integrally formed with the housing 51, ahinged portion laterally separated from the force contact 52, and a freeportion beyond the hinged portion that extends toward the top end of theforce contact 52.

The sense contacts 54 may be formed in layers and mounted to the topsurface of the housing 51, or on a membrane resting on the housing 51.For instance, the layer closest to the housing 51 may be a semi-rigidfilm-like layer, which is electrically non-conducting. Such a layer maybe formed from polyimide, kapton, PEEK, or any other suitable material.An electrically conducting layer may be deposited on top of thefilm-like insulator, and may be deposited in non-overlapping stripes,with each stripe corresponding to a particular terminal 2.

Such a layered structure for the sense contacts 54 may be used with anysuitable configuration for the force contacts 52, since there are noadded elements inside the housing directly between any of the forcecontacts 52. An example of force contacts 52 that may be used isdisclosed in U.S. Pat. No. 5,749,738, titled “Electrical interconnectcontact system”, and issued on May 12, 1998 to Johnson et al. Othersuitable force contacts 52 may be used as well.

Note that there may be some beneficial wiping of the terminal 2 from thesense contact 54 to reduce contact resistance due to oxide build up.Because the hinged portion is relatively close to the current contact,the free portion is relatively short compared to the vertical deflectionrange of the current contact. As a result, there is a significantlateral component to the vertical compression of the sense contact 54.In practical terms, this means that when the terminal 2 on the deviceunder test 1 initially contacts the sense contact 54, it makes contactat a particular location on the terminal 2. As the terminal 2 furtherdeflects/compresses the sense contact 54, the sense contact 54 slideshorizontally, though not sideways toward the force contact across theterminal 2. This sliding is generally considered beneficial, as it canbreak through any oxide layers that have built up on the terminal 2.

The specific geometry of the contacts determines the precise amount ofsliding. For a rigid free portion of length L that begins its travelextended upward by angle A and ends its travel flush with the housing(angle of 0), the horizontal extent of the wiping travel is L (1−cos A).Note that the vertical extent of the travel is L (sin A). In practicalterms, if the free portion is too long, then there is not enough lateraltravel to produce significant wiping. Likewise, if the free portion istoo short, then there is risk of damaging the free portion during use bybending or breaking an extending portion of the contact.

FIG. 6 shows another mechanical design 60 for the sense (voltage)contacts 64. Here, each sense contact 64 forms a fork with prongs thatextend onto opposite sides of the corresponding force contact 62. Thesense contact 64 prongs help keep the force contact 62 laterally alignedduring use by preventing or reducing lateral wobbling. By havingpotential connections on both sides of the force contacts 62, any devicemisalignment will result in making contact with at least one side of thefork.

In this case, away from the fork, the sense contact 64 is a solid,conducting member that lies on top of the test contactor housing 61. Thesense contact 64 extends horizontally away from the force contact 62,curves downward through a hole in the housing 61, emerges from thehousing 61 and contacts the corresponding contact pad 4 on the loadboard 3 as shown in FIGS. 13 and 14.

Each prong on the fork includes an upward-bending tip, which is angledpartially or fully toward the device under test 1. When the device undertest 1 is forced into contact with the tester 5, the terminal 2 contactsthe top end of the force contact 62 and the upward-bending tips on theprongs of the sense contact 64. The upward-bending tips may be rigid(with a well-defined angle that does not significantly vary during use),or may be spring-bendable. The upward bent tips also enables the sensecontact to avoid any protruding burrs on the device itself.

For the case of rigid (non-bending) tips, there is some wiping of oxidesfrom the terminal. The sharp points on the tips break through the oxideson the terminal. For the case of bendable tips, there may be appreciablewiping, in the manner described above with respect to FIG. 5. Asterminal 2 contacts the sense contact tips, the sense contact isdeflected vertically, allowing the horizontal portion of the sensecontact along the surface of the housing to flex. This provides thecontact force between the sense contact and terminal 2.

In the design 60 of FIG. 6, the housing 61 may include an inset in theregion around or near the force contacts 62, so that the sense contacts64 may be recessed slightly into the housing. Contacts 64 may include aplanar portion 64 a, a rising portion 64 b and a pair of fork tines 64c. The tines 64 c may have a sharp or pointed contact engagement surfacewhich will remove oxide from the terminal 2. Rising portion 64 b may belinear (a line) or follow a curved path to the tip 64 c. Tines 64 c mayhave triangular teeth as shown or other tapered or non taperedstructures. Preferably, tines 64 b surround on 2, 3 or 4 sides ofcontact 62 to help guide its alignment.

FIG. 11 shows a design 110 in which the sense contacts 114 may haveadditional resiliency provided by an elastomer material “pillow” 119 (ora cylinder 519 FIG. 22), disposed between the prongs of the sensecontacts 114 and the housing 111. Such a “pillow” 119 may provideadditional resiliency to the contacts, in addition to any existingresiliency from the force contacts 112.

FIG. 7 shows another forked design, in which the sense contacts areformed as layers, as is done with FIG. 5 above.

For the design 70 of FIG. 7, each sense (voltage) contact 74 has aportion 75 along the membrane or housing 71 which may or may not befixed and a free portion 76 extending hingedly away from the housing 71.There is a hinged portion 77 that connects the fixed portion 75 to thefree portion 76, with the hinged portion 77 being laterally separatedfrom the corresponding force (current) contact 72. The free portion 76has a forked portion 78 at its distal end that extends on opposite sidesof a distal end of the corresponding force contact 72. Note that in FIG.6 the free portion of contact 64 is longer than in FIG. 7 thus allowinggreater flexure and deflection. In other words, by moving the point offixation farther away from the tip, the flexure is increased, everythingelse being equal.

When the device under test 1 is forced toward the tester 5, thecorresponding terminal 2 on the device under test 1 simultaneouslycompresses the force contact 72 through the corresponding hole 73 in thehousing 71 and compresses the free portion 76 of the sense contact 74toward the housing 71.

As with the other designs shown herein, each terminal 2 on the deviceunder test 1 makes direct electrical and mechanical contact with a topend of the corresponding force contact 72. The terminal 2 on the deviceunder test 1 also makes direct electrical and mechanical contact withthe forked portion 78 of the corresponding sense contact 74. The forcecontact 72 does not make electrical contact with the sense contact 74,although both may mechanically and electrically contact the terminal 2on the device under test 1.

As with the design shown in FIG. 5, the fixed portion 75 may be platedonto the housing 71 or free floating and faces the device under test 1.When the sense contacts 74 are formed by such plating, each sensecontact 74 is generally planar, includes an electrically conductivelayer 79A facing the device under test 1, and includes an electricallyinsulating layer 79B facing away from the device under test 1.

For the forked design of FIG. 7, each sense contact 74 is generallyplanar, each forked portion 78 includes two parallel prongs, and eachprong includes a raised lip directly adjacent to the corresponding forcecontact 72 that extends out of the plane of the sense contact 74. Theraised or upwardly bent lip (downwardly in the leaded configurationbelow) may be formed by bending a rectangular portion of the prong outof its plane toward the device under test 1, the plane being definedadjacent portion of the sense contact which may be generally planar

For the exemplary design 70 of FIG. 7, along a dimension perpendicularto the forked portion 78 and parallel to the housing 71, thecorresponding terminal 2 on the device under test 1 is larger than thecorresponding force contact 72. This helps ensure that the device I/O orterminal 2 directly contacts both the force contact 72 and the sensecontact 74, because even if there is misalignment between the terminal 2and the contacts along the dimension cited above, the terminal 2 willdirectly contact at least one prong of the forked portion 78 of thesense contact 74, in addition to directly contacting the force contact72.

The design of FIG. 7 may allow for beneficial wiping of oxides from theterminal, as described above with respect to FIG. 5.

In FIG. 8, as the force (current) contact 82 is compressed over itsentire compression range, the sense (voltage) contact 84 remainsgenerally parallel to the housing 81 and can slide or translatelaterally along the housing 81. In this design, the sense contact 84completely laterally surrounds the force contact 82, so that if theforce contact 84 translates laterally, the sense contact 84 may follow.

More specifically, the sense contact 84 may translate to follow alateral cross-section of the force contact 82. Here are some clarifyingexamples. If the force contact 82 is completely cylindrical (i.e.,identical in lateral cross-section at each lateral plane, with eachlateral cross-section not necessarily being circular or elliptical), iscompletely longitudinally oriented, and compressed completelylongitudinally, then there is no lateral translation at all of the forcecontact 82, and the sense contact 84 does not move. If the force contact82 is cylindrical in shape, but is inclined with respect to thelongitudinal direction, and compresses purely longitudinally, then alateral cross-section of the force contact 82 does translate, and thesense contact 84 would follow such a translation and would laterallytranslate as well. If the force contact 82 is cylindrical in shape andhas a rotational component in its compression, as if there were anoff-axis pivot point to its compression, then there would be a lateralcomponent to its compression, with an extent determined by the pivotpoint location. If the force contact 82 is not a true cylinder in shape,then the sense contact 84 may follows its cross-section through changesin shape, size and/or orientation. For instance, the force contact 82may have a particular edge that advances or recedes laterally over thelongitudinal compression range, and the sense contact 84 may follow thatparticular edge for all or a portion of the compression range.

This lateral translation of the sense contact 84 may produce thebeneficial wiping of oxides off the terminal 2, as described above. Forinstance, the sense contact 84 may include a particular feature thatextends out of the plane of the sense contact 84, such as a prong, ashelf, a ledge, or an arm. This extending feature 85 may act like ablade at the terminal 2, and can help scrape through any oxide layersthat are present. It may also act as a guide to help maintain contact 82in alignment. In the exemplary design 80 of FIG. 8, the sense contact 84includes an arm bent upwards toward the device under test 1, with thearm being generally parallel to the adjacent face of the force contact82. Other suitable orientations are possible, as well.

In most cases, the lateral translation of the force contact lateralcross-section, over the range of longitudinal compression, is less thanthe size of the terminal 2 on the device under test 1, so that the forcecontact 82 does not “fall off” the device I/O or terminal 2 during use.

In the exemplary design 80 of FIG. 8, the sense contact 84 extendscompletely laterally around the force contact 82. Alternatively, theremay be one or more gaps in the sense contact 84, so that it extends onlypartially around the force contact 82. For instance, there may be a gapalong one or more sides, so that the sense contact 84 may still “catch”the force contact 82 to translate laterally. In some cases, the sensecontact 84 includes a partial or complete fork-like structure around theforce contact 82, and a partial or complete portion perpendicular to thefork prong that can engage one or both of the opposing sides of theforce contact 82 over its range of compression.

FIG. 9 shows a contact design 90 similar to that of FIG. 5, but using arelatively rigid rod 95 as the sense (voltage) contact 94. The force(current) contact 92 has a notch that accommodates an end of the sensecontact rod 95, so that the terminal 2 on the device under test 1 candirectly contact both the force contact 92 and the sense contact 94independently. An optional electrically insulating coating on the sensecontact 94 and/or the force contact 92 may help prevent the two contactsfrom shorting.

The rod 95 extends laterally away from the force contact 92, along theside of the housing 91 facing the device under test 1, then passesthrough a hole in the housing 91, exits the housing 91, and contacts arespective contact pad 4 on the load board 3. Note that any or allsections of the rod 95 may straight, may have some periodic or irregularcurvature, and/or may be coiled.

For the exemplary rod 95 shown in FIG. 9, the rod 95 does notsignificantly wipe through any oxide layers on the terminal.

A variant of the single rod of FIG. 9 is a dual-rod, shown in FIG. 10.

In the design 100 of FIG. 10, the sense (voltage) contact 104 includestwo rods 105, one on either side of the force (current) contact 102,that extend laterally away from the force contact 102 along the top sideof the housing 101. The rods 105 may join together at a point to form aforked portion, analogous to the forked structures shown above.Alternatively, the rods 105 may remain separate as they extend acrossthe housing 101. The rods 105 may pass through the housing 101 joined,through a single hole in the housing 101, or separately, throughindividual holes in the housing 101. Like many of the designs shownabove, having two sense contacts 104 on each side of the force contact102 adds redundancy in case of misalignment and also acts as aself-aligning tool to center the force contact 102 on the device I/O orterminal 2. Rods 105 preferably have a linear (straight) portion andthen a curved or angled portion extending generally orthogonally to thestraight portion.

In some cases, the rod or rods 105 may be sunk into a correspondingchannel or channels in the housing 101. Such channels may protect therods 105 from damage. Additionally, the channels may help attach therods 105 to the housing 101 or help in positioning rods in closeproximity to force contacts 102. In addition, because the rods 105 maybe electrically conductive, the housing 101 may be made from anelectrical insulator and may help electrically isolate each rod 105 fromthe other rods 105 and from other elements in proximity to the rods 105.Also, the rods 105 may be coated with an electrically insulatingmaterial to prevent shorting to their respective force contacts 102. Avariety of materials may be used, including parylene, Teflon®, Peek®,Kapton®, and so forth.

In some cases, each rod 105 has a distal end that bends out of the planeof the housing 101 toward the device under test 1. Such a bent distalend may improve electrical contact with the terminal 2 on the deviceunder test 1. Such a bent distal end may also localize the electricalcontact to the region near the bend, so that away from the bend, eachrod 105 is electrically insulated by the surrounding channel in which itresides.

In some cases, there is a pair of rods 105 associated with each forcecontact 102, with the pair of rods 105 being disposed on opposite sidesof the force contact 102. The rods 105 have distal ends, optionally bentout of the plane of the housing 101 toward the device under test 1, thatstraddle the force contact 102. The rods 105 then extend away from theforce contact 102 in the same direction along the housing 101,optionally within parallel channels in the housing 101. These parallelchannels may optionally be formed in a separate alignment plate thatmounts to the housing 101. The distal ends may also point toward a pointof convergence.

Along a dimension perpendicular to the rods 105 and parallel to thehousing 101, the corresponding terminal 2 on the device under test 1 islarger than the corresponding force contact 102. In general, when thedevice under test 1 is forced toward the housing 101, the correspondingterminal 2 on the device under test 1 simultaneously compresses theforce contact 102 through the corresponding hole 103 in the housing 101and contacts the distal end of at least one conductive rod 105 of thecorresponding sense contact 104.

In some cases, the rods 105 are directly adjacent to the force contact102. The rods may help hold the force contact 102 in place during useand may help prevent wobbling, which is beneficial.

In some cases, each rod 105 is an elongated cylinder, optionally with acircular cross-section. In other cases, each rod 105 may have arectangular or square cross-section. In some cases, each rod 105 may beformed separate from the housing 101, then attached to the housing 101or held in place by an alignment plate, which may be mounted on top ofthe housing 101. In other cases, each rod 105 may be made integrallywith the housing 101, such as by plating onto the surface of the housing101 or into a channel in the housing 101.

FIG. 13 is side-view cross-sectional drawing of a design 130, showing asample geometry of a sense (voltage) contact 134 in its path from theterminal 2 on the device under test to the contact pad 4 on the loadboard 3 which has a plurality of apertures 142 of predetermined gaptherein.

The contact 134 extends laterally away from the terminal 2 along a faceof the housing 131, bends roughly 90 degrees (orthogonal) to extendthrough a hole in the housing 131, (portion 134 b) and bends (portion134 c) generally equal to or preferably slightly less than 90 degrees tolie roughly parallel to the opposing face of the housing 131 throughaperture 142. This generally equal to or preferably less than 90 degreebend provides some bias force to the load board pad 4 assuring a solidconnection. When contacting the electrical contact pad 4 on the loadboard 3, a portion of the contact 134 is longitudinally disposed betweenthe contact pad 4 and the housing 131. Aperture 142 is sized to begreater than the thickness of the portion of the contact passing therethrough. In the preferred embodiment, the aperture is rectangular or thesame shape as the contact passing through, and the gap created betweenthe contact portion 134 b and the walls of the aperture should besufficiently great a turning force (lever action) can be transmittedfrom the force applied on contact 134 c/d by pad 4 (or 2) to contact 134on pad 2 (or 4). Thus, the gap is wide enough to control the position ofthe contact through the aperture but still transmit such force.Typically an aperture of twice or three times the thickness of thecontact portion will suffice.

Note also that the cross-sectional view may apply to any of the designsshown above in which the sense contact is generally planar (FIGS. 5-8and 11), or is generally a rod (FIGS. 9-10). In those cases where thesense contact is a self-supporting electrically conducting substrate,such as wire, or a metallic sheet, the substrate may be bent accordingto the geometry of FIG. 13. In those cases where the sense contact iscoated or plated onto an electrically insulating substrate, theinsulating substrate may be bent according to the geometry of FIG. 13.

In the specific design 130 of FIG. 13, both ends of the contact 134, arebent toward the terminal 2 on the device under test. There arealternatives to this geometry.

For instance, FIG. 14 shows a design 140 similar to design 130, in whichthe contact 144 extends laterally away from the terminal 2 along a faceof the housing 141, bends 90 degrees (portion 134 b) to pass through ahole 142 in the housing 141, and bends (portion 134 d) roughly equal toor preferably slightly less than 90 degrees to lie roughly parallel tothe opposing face of the housing 141. This generally equal to orpreferably less than 90 degree bend provides some bias force to the loadboard pad 4 assuring a solid connection. In contrast with the design 130of FIG. 13, the design 140 of FIG. 14 has opposite ends of the contact144 extending in opposite directions, rather than both ends extendingtoward the terminal 2

There may be potential advantages to the design 140 of FIG. 14, ratherthan that 130 of FIG. 13. For instance, the contact itself 144 may beeasier to fabricate and assemble. In some cases, such a contact 144 maybe easier to bend than the corresponding contact 134. In some cases, thegeometry of FIG. 14 may force the push pin into place (see turningforces above), thereby securing the assembled parts. In some cases, withthe geometry of FIG. 14, the torques generated by the terminal 2 or 4,with respect to the hole in the housing 141, may force the end of thecontact 144 into contact with the contact pad 4 or 2 on the load board3, which may be desirable. This provides a bias pushing the sensecontact towards the device under test and makes it easier to alignterminal 2 with sense contact 141.

The term “roughly parallel”, as used above, denotes that the 90 degreebend in the contact 134 and 144 directly adjacent to the contact pad 4may actually be less than 90 degrees. For instance, the bend may be inthe ranges of 70-90 degrees, 75-90 degrees, 80-90 degrees, 85-90degrees, 70-85 degrees, 75-90 degrees, 70-80 degrees, 75-85 degrees,80-90 degrees, 70-75 degrees, 75-80 degrees, 80-85 degrees, and/or 85-90degrees. In some cases, the bend angle may be 80 degrees.

Note that there may optionally be a radius to any or all of the bends inthe contact 134 and 144, rather than sharp angles, as drawn in FIGS. 13and 14. Such radii may simplify the manufacturability of the contact 134and 144.

Up to this point, the sense (voltage) contact has been shown generallyas a rod or set of prongs extending upward to the terminal 2 on thedevice under test 1. The ends of the sense contact may optionally havesome structure to them, which in some cases may assist with the wipingfunction described above. Some examples are shown in FIGS. 15-20.

FIG. 15 is a side-view schematic drawing of a pair of straddling sensecontacts 154 having tips 155 that are angled away from the central force(current) contact 152. The angling may be confined to within the planeof the page, or may optionally extend out of or into the plane of thepage.

FIG. 16 is a top-view schematic drawing of a pair of sense contacts 164that, at their distal ends 165, include laterally-extending portions 166that extend toward each other converging to a distant point. The sensecontacts 164, including their laterally-extending portions 166, surroundthe central force contact 162.

In addition, the laterally-extending portions 166 extend out of theplane of the page, toward the terminal 2 on the device under test 1 (notshown). In the view of FIG. 16, the terminal 2 would reside between theplane of the page and the viewer. The tips 167 of the laterallyextending portions 166 would be closer to the viewer than the rest ofthe contacts 164. Notice that the contacts 166, 167 extend generallyorthogonally away from arms 165 so that they are parallel to each otherand would be intersected by a longitudinal axis 169 drawn throughcontact 162.

FIG. 17 is a top-view schematic drawing of a pair of sense contacts 174that, at their distal ends 175, include laterally-extending portions 176that extend toward each other and generally orthogonal to their arms174. The sense contacts 174, including their laterally-extendingportions 176, do not surround the central force contact 172 but would beintersected by a longitudinal axis 169 passing through 172. One maythink of the central force contact 172 as being “outside” a polygonformed by the pair of sense contacts 174 and their laterally extendingportions 176.

As with FIG. 16, the laterally-extending portions 176 extend out of theplane of the page, the terminal would reside between the plane of thepage and the viewer, and the tips 177 of the laterally extendingportions 176 would be closer to the viewer than the rest of the contacts174.

FIG. 21 is a top-view schematic drawing of a pair of sense contacts 214that, at their distal ends 215, include laterally-extending portions 216that extend toward each other. The sense contacts 214, including theirgenerally orthogonal laterally-extending portions 216, surround thecentral force contact 212, which protrudes through the gap betweencontacts 214 and 215. The tips 217 of the laterally extending portions216 would be closer to the viewer than the rest of the contacts 214.Here, the laterally extending portions 216 lie on opposite sides of theforce contact 212.

FIG. 18 is a top-view schematic drawing of a single sense contact 184that, at its distal end 185, includes a laterally-extending portion 186that extends partway around the force contact 182. As with FIGS. 16 and17, the laterally-extending portion 186 extends out of the plane of thepage, the terminal would reside between the plane of the page and theviewer, and the tip 187 of the laterally extending portion 186 would becloser to the viewer than the rest of the contact 184.

FIG. 19 is a top-view schematic drawing of a single sense contact 194that, at its distal end 195, includes a laterally-extending portion 196that does not extend partway around the force contact 192. The laterallyextending portion 196 is on the opposite side of the force contact 192as the laterally extending portion 186 shown in FIG. 18. As with FIGS.16-18, the laterally-extending portion 196 extends out of the plane ofthe page, the terminal would reside between the plane of the page andthe viewer, and the tip 197 of the laterally extending portion 196 wouldbe closer to the viewer than the rest of the contact 194.

FIG. 20 is a side-view schematic drawing of a pair of sense contacts 204having tips 206 that are angled toward each other and cross each otherover or alongside the central force contact 202. The tabs or tips 206can cross the force contact 202 behind or in front of the force contact202. During contact with the terminal 2 on the device under test, theterminal pushes downward on the tips 206, forcing them away from eachother, creating a scrubbing action to break through oxide layer onterminal 2. Elimination of oxides is an important outcome in thisembodiment. The angling may be confined to within the plane of the page,or may optionally extend out of or into the plane of the page. The crossover arms may have a non-conductive coating at least on their back sideto prevent shorting to the contact 202 or contact 202 may just be formedso that it cannot contact tips 206.

In general, the sense contacts may have tips or tabs that may extend outof the plane of the membrane, toward the terminal on the device undertest. When contacting the terminal, the tabs may bend or flexindependently of the motion or orientation of the rest of the sensecontacts. This motion may be a bending motion, such as with a generallyflexible material, and may optionally include a hinged structure at theproximal end of the tab that joins it to the rest of the sense contact.The tabs may optionally extend laterally toward, across or around theforce contact. In some cases, there is a single sense contact that has atip that extends laterally past the force contact, so that the forcecontact is partially “inside” or partially “outside” the confines of thesense contact. In other cases, there are two sense contacts that aregenerally parallel to each other, with tips that extend toward eachother, so that the force contact is partially “inside” or partially“outside” the confines of the two sense contacts. Alternatively, thetips may extend laterally away from each other, or may extend in anygeneral lateral direction, in addition to extending out of the planetoward the terminal on the device under test.

Although the force (current) contact is typically thicker than the sense(voltage) contact, and the above description has assumed such, it shouldbe noted that the functions of the two contacts may be reversed, so thatthe thinner contact carries the current and the thicker contact measuresthe voltage. The preferred application of this would be to make thecontact in housing slot thinner and make the contact on top of housingthicker to handle more current.

In addition to ball grid array (BGA) and other leadless packages withpads on the underside of device, the present construction may also beapplied to components in particular integrated circuits with leads orwires, known as leaded-contact packages.

FIGS. 22-32 illustrate Kelvin contacts for leaded packages. To theextent that elements for BGA packages or packages with pads on theunderside of device are similar, they will have the same part numberincreased by 500. Therefore, contacts 2 in BGA appear as contacts 502 inleaded, and so forth.

FIGS. 22, 22, 23, and 24 shows a leaded device (DUT) 501 with aplurality of leads 502 a each having contacts 502. As in the case of padpackages, a force contact 552 makes contact with a lead 502, usually ina central portion thereof. Contact 552 is upwardly biased by element 519similar to pillow 119 (FIG. 11) but preferably cylindrical. (Note thatthe cylindrical form may also be used in pad or BGA constructions.) Asecond bias block 519 a is used to apply a downward force on the rockingpin 600. Rocking pin 600 is similar to that shown in U.S. Pat. Nos.5,069,629 and 7,445,465 and is hereby incorporated by reference.

Contact extensions 544 are formed as shown in FIG. 22 so that theyfollow a path to a load board 503, where they make electrical contact.The extension provides easier load board layout and facilitates easiertrace routing on the load board. The sense contact tips in FIG. 22 havea dual tine fork design. In this case the fork tip are flat horizontallywith no upward curvature like in FIG. 6. This allows the terminal leadedge of the device under test to wipe along the top surface of the forkand remove oxides.

FIGS. 25, 26, 27, 28, 29, 30, 31, and 32 provide greater detail as tohow the force 552 and sense contacts 554 function for three differentconcepts. Force Contact in FIGS. 27, 28, and 29 has a full width forcecontact 600. Force contact in FIGS. 25, 26, 30, 31, and 32 have a tip552 that is reduced in thickness to prevent shorting to sense contact554 and to provide a more knife edge (ie tip narrower than the base) topenetrate oxide layers on devices under test leads. In FIG. 31, a ledge620 is shown which is a portion where the tip 552 is narrowed. FIG. 27shows an alternate approach to FIG. 26 in that the end of sense contact554 is raise out of the page similar to concept shown in FIGS. 20 and21. First the force contact tip 552 preferably has a “toothlike crown”with a dip or recess 552 a. The sense contacts 554 in FIGS. 25, 26, 30,and 31 have a forked end terminating in two tines 554 a and 554 b,which, at their distal ends have tines which are angled downwardly(opposite of pad package tines 554 a) at 20-30 degrees (ie 20, 21 . . .or 30 degrees) off the plane of contact 554. FIG. 32 shows an alternateapproach to dual tine approach in that the force contact tip 552 isoffset to one side and sense contact 554 has only one tine. The gapbetween the force contact tip and sense tine is centered on device leadcenterline to assure adequate contact. The distal end of contact 554 maybe chamfered or rounded on its lower surface to provide additionalclearance. The inner periphery 602 of the force contact 600 is cut away(i.e. the thickness front to back is reduced) to insure clearance withsense contact 554 as it moves.

In operation, contact 600 “rocks” from the two positions shown in FIG.29 in response to contact with leaded contact 502. Likewise the sensecontact 554 moves between the two positions shown. The movement of thevarious components in FIG. 30 is the same as FIG. 29 except that FIG. 30shows sense contacts with downwardly angled tines and reduce tip widthcontact 600. Note that the tines need not be downwardly angled tofunction.

FIG. 32 shows a variation on the dual tine construction of FIG. 31. Inthis case the sense contact 554 has only a single tine 554 a which mayor may not have a downwardly angled portion (such as shown in FIG. 31).This allows for the force contact to have a greater contact surface areaif desired. In this design the force contact tip is offset and notcentered on the contact 600 width.

FIG. 33 provides clarity in the following ways. In this embodiment, thesense contact 554 is not forked. Further, the sense and force contactsare preferably collinear but preferably never come into contact witheach other. It is possible for the two to contact each other brieflyduring insertion if so configured FIG. 33 shows the leaded contact 502in two positions. The rearmost is where first contact is made. The sensecontact 554 first encounters the lead 502 and as 554 is compressed byits own resilience or alternatively against the resilient element 519 a,there will be a wiping action between the two. As shown in theforeground contacts, the sense contact 554 is compressed downwardlyuntil it resides behind and adjacent the rocking force contact 552. Itis apparent that they are collinearly aligned at all times. There isalso a wiping action between contact 502 and 552 as 552 rocks inresponse to elastomer 519

The disclosure also inherently includes a method of constructing adevice according to the disclosure. In addition there is a method ofminimizing contact resistance when conducting temporary contact betweena device under test and a test fixture having test contacts. Minimizingresistance is the goal of the wiping action described above. The testfixture has a force and sense fixture contact, to receive the testcontact, and involves at least some or all of the following steps:

a. collinearly aligning the sense and force contact

b. resiliently locating the sense contact in a plane above the forcecontact but lateral thereto,

c. bring the test contact into physical contact with the sense contact

d. deflecting the sense contact by the test contact, causing the sensecontact to wipe the test contact during deflection

In addition the method may also include the following steps or partsthereof: allowing the force contact to be deflected when it encountersthe test contact by configuring the force contact to have a rockingresponse to impingement.

The description of the invention and its applications as set forthherein is illustrative and is not intended to limit the scope of theinvention. Variations and modifications of the embodiments disclosedherein are possible, and practical alternatives to and equivalents ofthe various elements of the embodiments would be understood to those ofordinary skill in the art upon study of this patent document. These andother variations and modifications of the embodiments disclosed hereinmay be made without departing from the scope and spirit of theinvention.

We claim:
 1. A device for forming a plurality of temporary mechanicaland electrical connections between a device under test (1) having aplurality of terminals (2, 502), comprising: a plurality of electricallyconductive force contacts (72, 552) extending toward a device under test(1, 501) and being deflectable, each force contact (72, 552) in theplurality being laterally arranged to correspond to one terminal (2,502); and a plurality of electrically conductive sense contacts(74,554), each sense contact (74, 554) in the plurality being laterallyarranged to correspond to one force contact (72, 552) and one terminal(2, 502), each sense contact (74, 554) in the plurality extending towardthe device under test (1, 501) proximate the corresponding force contact(72, 552); wherein each sense contact (74, 554) in the pluralityincludes a freely movable portion (76, 554) extending resiliently towarddevice under test (1,501) wherein the sense contact is laterallyseparated from the corresponding force contact (72, 552); and whereinthe free portion includes a forked portion (78, 554 a,b,c,d) at itsdistal end that extends on opposite sides of a distal end of thecorresponding force contact (72, 552).
 2. The device of claim 1, whereinwhen the device under test (1, 501) is forced downwardly thecorresponding terminal (2, 502) on the device under test (1, 501)simultaneously compresses the force contact (72, 552) and compresses thefree portion of the sense contact (74, 554).
 3. The device of claim 1,wherein each terminal (2, 502) in the plurality makes direct electricaland mechanical contact with a distal end of the corresponding forcecontact (72, 552); and wherein each terminal (2) in the plurality makesdirect electrical and mechanical contact with the forked portion (78,554 abcd) of the corresponding sense contact (74, 554).
 4. The device ofclaim 1, wherein each sense contact (74, 554) in the plurality: isgenerally planar, includes an electrically conductive layer (79A) facingthe device under test (1, 501), and includes an electrically insulatinglayer (79B) facing away from the device under test (1).
 5. The device ofclaim 4, wherein the lip is formed by bending a rectangular portion ofthe tine out of its plane toward the device under test (1, 501).
 6. Thedevice of claim 1, wherein each sense contact (74,554) in the pluralityis generally planar, and wherein each forked portion (78, 554 abcd)includes two parallel tines; and wherein each tine includes a lipdirectly adjacent to the corresponding force contact (72) that extendsout of the plane of the sense contact (74).
 7. The device of claim 1,wherein along a dimension perpendicular to the forked portion (78, 554)the corresponding terminal (2) on the device under test (1, 501) islarger than the corresponding force contact (72, 552).
 8. The device ofclaim 1, wherein the forked portion (78 554) includes tips at its distalend that extend laterally at least partially converging toward eachother.
 9. The device of claim 8, wherein the tips cross each other. 10.The device of claim 1 wherein said sense contact (554) and said forcecontact (552) are collinearly aligned.
 11. The device of claim 1 whereinsaid sense (554) contact is positioned to make first contact with saiddevice terminal (502) when said device under test is applied thereto,and thereafter said force contact (552) makes contact with saidterminal.
 12. The device of claim 1 wherein said sense (554) contact ispositioned above and spaced laterally apart from said force contact(552) while positioned collinearly therewith, so that the sense contactmakes first contact with said device terminal (502) when said deviceunder test is applied thereto, and thereafter said force contact (552)makes contact with said terminal.
 13. The device of claim 1 wherein saidsense contact (554) is resiliently biased against said terminal (502)and slides there-across as the terminal in pressed against said sensecontact by virtue of the angular movement of the sense contact.
 14. Thedevice of claim 1 wherein said sense contact includes a distal forkedend including a pair of tines (554 ab).
 15. The device of claim 14wherein said tines are angled downward.
 16. The device of claim 14wherein said tines are angled upward.
 17. The device of claim 1 whereinsaid force contact (552) includes a tip, and wherein said tip includes arecess (552 a) therein.
 18. The device of claim 1 wherein said forcecontact (552) is collinearly aligned with said sense contact (554) andwherein the force contact is formed so that the sense contact with notimpinge thereon when the terminal deflects the sense contact downwardly.19. The device of claim 1 wherein the force contact (552) has a tipwhich is thinner than its base, and wherein the tip passes through apair of tines (554 a-b) which surround the tip on at least two sides.20. The device of claim 1 wherein the force contact (552) has a tipwhich is thinner that its base and wherein the tip passes adjacent asingle tine (554 c) on the sense contact (554.
 21. A device for forminga plurality of temporary mechanical and electrical connections between adevice under test (1) having a plurality of terminals (2) and a loadboard (3) having a plurality of contact pads (4), each contact pad (4)being laterally arranged to correspond to exactly one terminal (2),comprising: a laterally oriented, electrically insulating housing (71)longitudinally adjacent to the contact pads (4) on the load board (3); aplurality of electrically conductive force contacts (72) extendingthrough longitudinal holes (73) in the housing (71) toward the deviceunder test (1) and being deflectable through the holes (73) in thehousing (71), each force contact (72) in the plurality being laterallyarranged to correspond to exactly one terminal (2); and a plurality ofelectrically conductive sense contacts (74), each sense contact (74) inthe plurality being laterally arranged to correspond to exactly oneforce contact (72) and exactly one terminal (2), each sense contact (74)in the plurality extending toward the device under test (1) proximatethe corresponding force contact (72); wherein each sense contact (74) inthe plurality includes a fixed portion (75) along the housing (71), afree portion (76) extending hingedly away from the housing (71), and ahinged portion (77) connecting the fixed portion (75) and the freeportion (76); wherein the hinged portion (77) is laterally separatedfrom the corresponding force contact (72); and wherein the free portion(76) includes a forked portion (78) at its distal end that extends onopposite sides of a distal end of the corresponding force contact (72).22. The device of claim 21, wherein the fixed portion (75) is platedonto the housing (71) and faces the device under test (1).
 23. Thedevice of claim 21, wherein each terminal (2) in the plurality makesdirect electrical and mechanical contact with a distal end of thecorresponding force contact (82); and wherein each terminal (2) in theplurality makes direct electrical and mechanical contact with an angledportion (85) of the corresponding sense contact (84) that extends towardthe device under test (1).
 24. The device of claim 23, wherein theangled portion (85) is fixed.
 25. The device of claim 23, wherein theangled portion (85) is deflectable toward the housing (81).
 26. Thedevice of claim 23, wherein the angled portion (85) is formed only fromthe electrically conductive layer (86A).
 27. The device of claim 21,wherein each sense contact (84) in the plurality: is generally planar,includes an electrically conductive layer (86A) facing the device undertest (1), and includes an electrically insulating layer (86B) facingaway from the device under test (1).
 28. The device of claim 21 whereineach sense contact (84) in the plurality includes an angled portion (85)directly adjacent to the corresponding force contact (82) that extendsout of the plane of the sense contact (84).
 29. A method of minimizingcontact resistance when conducting temporary contact between a deviceunder test (DUT) and a test fixture having test contacts, the testfixture having at least one force and sense contact, to receive the DUTtest contact, the method comprising: a. collinearly aligning the senseand force contact; b. resiliently locating the sense contact in a planeabove the force contact but lateral thereto, c. bring the DUT testcontact into physical contact with the sense contact; d. deflecting thesense contact by the DUT test contact, causing the sense contact to wipethe DUT test contact during deflection.
 30. The method of claim 29further including the steps of allowing the force contact to bedeflected when it encounters the DUT test contact by configuring theforce contact to have a rocking response to impingement.
 31. The methodof claim 29, wherein when the device under test (DUT) (1) is forcedtoward the housing (101), the corresponding terminal (2) on the deviceunder test (1) simultaneously compresses the force contact (102) throughthe corresponding hole (103) in the housing (101) and contacts thedistal end of at least one conductive rod (105) of the correspondingsense contact (104).
 32. A device for forming a plurality of temporarymechanical and electrical connections between a device under test (1,501) having a plurality of terminals (2,502) and a load board (3) havinga plurality of contact pads (4), each contact pad (4) being laterallyarranged to correspond to exactly one terminal (2), comprising: alaterally oriented, electrically insulating housing (81) longitudinallyadjacent to the contact pads (4) on the load board (3); a plurality ofelectrically conductive force contacts (82) extending throughlongitudinal holes (83) in the housing (81) toward the device under test(1) and being deflectable through the holes (83) in the housing (81),the compressibility including a lateral translation of a lateralcross-section of each force contact (82), each force contact (82) in theplurality being laterally arranged to correspond to exactly one terminal(2); and a plurality of electrically conductive sense contacts (84),each sense contact in the plurality being laterally arranged tocorrespond to exactly one force contact (82) and exactly one terminal(2), each sense contact (84) in the plurality laterally surrounding thecorresponding force contact (82) and being laterally slidable along thehousing (81), the lateral sliding corresponding to the lateraltranslation of the lateral cross-section of the corresponding forcecontact (82).
 33. The device of claim 32, wherein when the device undertest (1) is forced toward the housing (81), each terminal (2) in theplurality simultaneously compresses the corresponding force contact (82)through the corresponding hole (83) in the housing (81) and laterallyslides the corresponding sense contact (84) along the housing (81). 34.The device of claim 32, wherein along a dimension parallel to thesliding of the corresponding sense contact (84), the correspondingterminal (2) on the device under test (1) is larger than thecorresponding force contact (82).
 35. A device for forming a pluralityof temporary mechanical and electrical connections between a deviceunder test (1) having a plurality of terminals (2) and a load board (3)having a plurality of contact pads (4), each contact pad (4) beinglaterally arranged to correspond to exactly one terminal (2),comprising: a laterally oriented, electrically insulating housing (101)longitudinally adjacent to the contact pads (4) on the load board (3); aplurality of electrically conductive force contacts (102) extendingthrough longitudinal holes (103) in the housing (101) toward the deviceunder test (1) and being deflectable through the holes (103) in thehousing (101), each force contact (102) in the plurality being laterallyarranged to correspond to exactly one terminal (2); and a plurality ofelectrically conductive sense contacts (104), each sense contact (104)in the plurality being laterally arranged to correspond to exactly oneforce contact (102) and exactly one terminal (2); wherein each sensecontact (104) in the plurality includes a pair of electricallyconductive rods (105) extending generally laterally along the housing(101); wherein the pair of electrically conductive rods (105) fit withincorresponding channels in the electrically insulating housing (101);wherein each conductive rod (105) in the pair has a distal end bent outof the plane of the housing (101) toward the device under test (1); andwherein the two distal ends in each pair of rods (105) are directlyadjacent to and are on opposite sides of the corresponding force contact(102).
 36. The device of claim 35, wherein each force contact (102) inthe plurality transmits current; and wherein each sense contact (104) inthe plurality measures voltage.
 37. The device of claim 35, whereinalong a dimension perpendicular to the rods (105) and parallel to thehousing (101), the corresponding terminal (2) on the device under test(1) is larger than the corresponding force contact (102).
 38. The deviceof claim 35, wherein the two distal ends in each pair of rods extendlaterally toward each other.
 39. The device of claim 35, wherein the twodistal ends in each pair of rods cross each other.